Propanil is a herbicide that is used extensively on rice and wheat to control weeds. Our preliminary data indicates that propanil has a profound immunotoxic effect on acutely dosed mice. 3,4,3',4'- tetrachloroazobenzene (TCAB), a contaminant of commercial propanil and a metabolic breakdown product of propanil, has a similar structure to that of the well studied dioxins that have been shown to have potent immunotoxic properties. Thus, it would appear prudent that the immunotoxic properties of propanil, TCAB and other possible metabolites be extensively studied. This will be accomplished by initially determining the effects of these xenobiotics of the humoral and cell-mediated immune responses using the Tier I and II testing protocols recommended by the National Toxicology Program (NIP). Extensive experimentation is planned or follow these initial studies to determine the mechanism of the immunotoxicity at the molecular level.